1. Field of the Invention
The present invention relates to a phase shift mask to be used in a photolithogaraphy step of semiconductor fabrication and an inspection method thereof, and more particularly to an inspection method for measuring transmittance error and phase shift angle error of a phase shifter and a phase shift mask used therefor.
2. Description of the Related Art
Phase-shifting technology has been expected as one of useful method to enhance the performance of optical lithography. It is necessary to control with high precision transmittance and phase shift angle of the phase shifter to obtain good results. Heretofore, the measurement of transmittance error of a phase shifter of the mask has been measured by, for example, a self-spectrophotometer having light beam diameter of about 5 mm. The transmittance error is defined as the percentage of drop in the transmittance of the transparent portion where the phase shifter is placed when the transmittance of the transparent portion where the phase shifter is absent is taken as 100%. In addition, regarding the measurement of the phase shift angle of the phase shifter, there is proposed a method of mechanically measuring the film thickness of the phase shifter by, for example, a profilometer in which a needle is brought into contact with the phase shifter to trace the thickness, then calculating the phase shift angle, or a method which is disclosed in A. P. Ghosh and D. B. Dove, "Direct Phase Measurements in Phase Shift Masks", SPIE, Vol. 1673 (1992), pp. 242-249. The proposed phase shift angle measurement is shown in a sectional diagram in FIG. 1. Light beam emitted by a He-Ne laser 7 is split by a half mirror 8 into light beam B1 which is transmitted through a phase shifter 3 of a phase shift mask (consisting of a glass substrate 1, a chromium film 2 for forming a pattern, and a phase shifter 3) and light beam B2 which is transmitted through the adjacent portion where the phase shifter is absent. Then, light beams B1 and B2 are focused by a lens 9 on the surface of the phase shift mask. Finally, the two light beams B1 and B2 that are transmitted through the phase shift mask are combined again by a lens 9 and a half mirror 8, their interference intensity is determined by an optical interferometer 10, and the phase shift angle error is determined by calculating the phase difference from the value of the interference intensity. The phase shift angle error is defined as the deviation from the phase shift angle of 180.degree. at which the maximum effect of the phase shift method can be obtained. In this case, the error is taken positive if the phase shift angle is greater than 180.degree., and it is taken negative if the phase shift angle is smaller than 180.degree..
In these conventional inspection methods of measuring the transmittance error of the phase shifter, when the transmittance error is measured using a self-spectrophotometer, the beam diameter is about 5 mm, so that there is required a pattern with greater than 5 mm in diameter for monitoring transmittance error of the phase shifter. When this requirement is applied to the phase shift mask which is to be mass produced, it is difficult to accommodate the pattern for measurement due to the problem of layout space.
Moreover, in the conventional inspection method of measuring the phase shift angle error of the phase shifter, when the phase shift angle error is measured using a profilometer, there is required a needle whose radius of curvature at the tip is a minimum of about 10 .mu.m and a pattern with area greater than 100 .mu.m square for measurement of the phase shift angle error of the phase shifter, considering the moving range of the needle centered around a step. In addition, in this case, the phase shift mask is damaged because the measurement is made by mechanically bringing the needle into contact with the phase shifter, which result in the deterioration in the yield of manufacture of the phase shift mask. For this reason, it was necessary to manufacture a dummy mask for monitoring for every regular phase shift mask.
Furthermore, when the phase shift angle error is measured using an optical interferometer, it is also necessary to arrange a broad pattern on the phase shift mask analogous to the case of using the self-spectrophotometer and the profilometer. In addition, in order to sufficiently enjoy the performance of the excellent resolution and the large focus depth of the phase shift mask, it is necessary to control with high precision the transmittance and the phase shift angle of the phase shifter.